Darchem Thermal Protection

Darchem Thermal Protection 1

Radiant Heat and Wind Shield Solutions Darchem Thermal Protection, a strategic business unit within Darchem Engineering, specialises in the design, development and manufacture of Radiant Heat and Wind Shields for the protection of personnel, equipment and structures from radiant heat, fire and inclement weather conditions. Darchem offers two types of Heat Shield solutions based upon the heat shield location, the incidental heat flux levels and the required reduction in thermal radiation. Darscreen™

Darscreen™ heat shields are based on a woven mesh design and are intended for use with lower level heat fluxes, or for wind shielding. It is commonly employed as a vertical heat shield protecting critical equipment, structures, personnel, muster points and escape routes in areas located away from flare stacks, but still exposed to unacceptable radiation levels.

Darflare™

The Darflare™ ‘Chevron System’ is designed for use with higher heat flux levels such as those experienced during flaring operations, and is often installed as part of the flare platform structure.

Darscreen™ wind / heat shield protection for escape stair tower

Darflare™ - BP Bruce, UK North Sea Darflare™- prior to site installation

Radiant Heat Shielding

Darchem Thermal Protection 2

The science of Thermal Radiation Thermal radiation is the transmission of heat by the propagation of electromagnetic radiation. Unlike other forms of heat transfer it does not increase and decrease linearly with temperature but proportionately to the 4th power of the absolute temperature. The relationship between temperature and thermal radiation is defined by the relationship:

q = ε σ AT4

Radiant heat flux is equal to the product of emissivity, Stefan-Boltzmann constant, area and the fourth power of absolute temperature

Thermal radiation forms part of the electromagnetic spectrum of which visible light, radio waves and microwaves are also part. Thermal radiation is generally referred to as heat flux and is measured in kW/m². In general, thermal radiation can be compared to the behaviour of visible light; in that it emanates from a point or series of points and disperses equally in all directions. Therefore, the intensity of the heat will diminish as the distance from the heat source increases. Radiant Heat Shielding design considerations Thermal Radiation Shielding can be divided in to two distinct categories:- a) Continuous Shielding - as in the case of a flare tip on an Offshore Installation

b) Fire Hazard Shielding (limited duration) - to limit exposure where a piece of equipment is

adjacent to a potential fire hazard. As there is currently no international standard, Heat Radiation Shielding design is substantiated by extensive testing, witnessed by 3rd party verifying bodies such as Lloyds and DNV. Each heat shield manufactured is individually designed based on the incidental heat flux levels and frequency, heat reduction target and any additional customer specific requirements. Darscreen™

The Darscreen™ woven mesh system is designed for lower heat flux levels. Although the system can be employed as a horizontal heat shield fitted directly below the flare tip, it is more commonly used as a vertical heat shield solution protecting critical plant equipment, personnel, and escape routes in the event of a fire. Darscreen™ can be installed to offer up to a 95% reduction in radiated heat.

Darscreen™ heat shields – Chevron Escravos Platform, Nigeria

Radiant Heat Shielding

Darchem Thermal Protection 3

Darscreen™ is manufactured as a modular system with individual panels made up of a 1 or 2 layers of 4 x 4, or 8 x 8 woven mesh fixed to a box section frame. Where a load bearing requirement is present, i.e. in a horizontal system, grating can be welded to the top edge of the frame. The individual panels are kept at a manageable size to allow ease of installation. Where solid system barriers would prevent natural lighting and could trap gas the woven mesh system has excellent see-through properties and is naturally venting. In comparison with a solid barrier Darscreen™ places less loading from wind pressure on the supporting structure. Wind Shielding Darscreen™ can also be installed to protect personnel using walkways, stairwells, and other areas exposed to high wind conditions. The use of a mesh system has the advantage over a perforated plate system by having better visibility for personnel and less risk of stagnant regions.

The loading on the structure is less and the wind reduction is more evenly distributed than that achieved with perforated plate.

A choice of mesh size is available to achieve the required wind reduction for the lowest cost. Darscreen™ Radiation Shield Performance Darchem can predict the reduction in radiation heat flux for various different configurations of Darscreen™ woven mesh heat shields. For a known incident heat flux on the hot face of the heat shield the attenuated heat flux can be predicted at set distances from the cold face of the heat shield. A spreadsheet has been created using comprehensive test data on vertical heat shields. The following summary shows the predicted heat flux in kW/m2 at a distance X from the cold face of the heat shield for various different shield designs. In conjunction with the recommended exposure data, it can be seen that in this example, input heat flux of 8.5 kW/m2 and X = 100mm, a Single Layer 8 x 8 woven mesh shield would be necessary to reduce the stated input heat flux to an acceptable level for general areas where personnel may be continuously exposed. For emergency escape-ways, a Single Layer 4 x 4 panel design would be acceptable.

Model showing mounting of Darscreen™

Radiant Heat Shielding

Darchem Thermal Protection 4

Radiant Heat Shielding

Darchem Thermal Protection 5

Darflare™ The Darflare™ ‘chevron’ system was developed in the early 1980’s in collaboration with British Petroleum after experiencing problems on the Magnus Offshore Platform. The flare tip heat flux was higher than anticipated and the heat shield installed was buckling and distorting under normal operating conditions. As a result the underlying steelwork was being compromised due to being exposed to temperatures beyond acceptable design conditions. Following the Darchem / BP Collaboration the Darflare™ was

developed and installed on the Magnus Platform. This not only resolved the problem but also offered a heat flux reduction of more than 90%.

Darflare™ panels are designed to allow convective airflow through the assembly, thereby reducing the radiant heat on the cold face of the shield to below 100°C during flaring. This ensures that the structural steelwork is not compromised. The system is manufactured as a modular design with each panel made up of a fabricated perimeter consisting of a metallic edge skin with a load bearing supporting grating fixed to the top surface ‘hot face’ of the panel. Formed thin sheet foils provide a contoured surface to promote cold air convection up through the panel to the ‘hot face’.

The bottom surface ‘cold face’ of the panel is supported by expanded metal to facilitate airflow and to hold the foil sheets in place.

Darflare™ ‘chevron’ system installed on the BP Magnus Platform, UK North Sea

Darflare™ cross-section showing the load-bearing grating and underlying contoured foils.

Diagram showing the cold air convection though to the hot face of the Darflare™ - hence the ‘chevron’ description

Completed panel sections incorporating grating and foils

Radiant Heat Shielding

Darchem Thermal Protection 6

The panels are assembled with a proprietary fixing system, designed to allow the system to expand during flaring intervals while holding each panel in place to the supporting structure. To allow access for maintenance, hatchways can be incorporated in to the system, these are constructed using hinged access doors secured with shoot bolts, welded to the cold face of the shield.

Principle Features of the Radiant Heat Shield Systems Weight: Darflare™ heat shields have a weight of between 50 to 60Kg/m2 depending

on the requirement for load bearing. Darscreen™ - mesh panels range between a weight of 20 to 40Kg/m2, depending on the chosen design

Materials: Darchem heat shields can be designed and manufactured using either grade 316 stainless steel or high nickel alloys.

Maintenance Free: The Heat Shields are designed to be completely maintenance free during the projected life of the platform, even when subjected to thermal cycling from the operational flares and extreme environmental conditions.

Heat Dissipation: Darflare™ and Darscreen™ have greater heat dissipation than solid barriers. In comparison with the woven wire mesh, a solid plate barrier would have a potentially dangerous higher touch surface temperature.

See-through venting:

The Darscreen™ woven wire system has excellent see-through properties and is naturally venting. Solid systems prevent natural light and could trap gas that may permeate from the adjacent process plant. The see-though properties reduces the claustrophobic effect that a solid system may present during normal operating conditions.

Darchem Thermal Protection, Darchem Engineering Ltd, Stillington, Stockton-On-Tees, TS21 1LB, UK

Tel +44 1740 630461 – Email dtpsales@darchem.co.uk – Website www.darchem.co.uk

Darflare™ heat shield model – overall design showing panel arrangement, access points and hand rail & pulling posts

Darflare™ heat shield installed on the Chevron Benchamas platform, Thailand.

Revision B March 2013